The present invention relates to a method for operating an electromotive drive as well as to an electromotive drive for carrying out such method.
When using a contactlessly mounted rotor, which is configured as a rotor of an electromotive drive revolving at a high rotational speed, it is of particular interest to prevent the rotor contacting the catch bearings due to external process and disturbing forces acting on it. The rotor contacting the catch bearings involves the risk that the kinetic energy of the rotor being released will lead to premature wear of the catch bearings through to mechanical destruction of the catch bearings and/or the entire inner drive assembly.
Knowledge of the spacing of the bearing air gaps is sensible in particular in the case of occurring process and disturbing forces in order to promptly initiate switching off of the electromotive drive before contact between the rotor and catch bearing. A disturbing force of this type may result, in rotors which are used for textile machines, for example, from a thread accumulation being deposited on the shaft of the rotor between the bearing magnets and exerting an axially directed force on the rotor and displacing it from the force-free floating state occurring in stationary operation in the direction of one of the catch bearings. This disturbing force is counteracted by corresponding activation of a control device, with the result that with a lack of knowledge of the remaining bearing air gap, the rotor may be pressed against one of the catch bearings and this may result in damage to the drive.
It is known from the published application German Patent Publication DE 35 23 344 A1, to use gap sensors which measure absolutely to determine the bearing air gap of a contactlessly mounted rotor of an electromotive drive, which sensors are in a position to precisely indicate, without delay and by measurement techniques gap deviations of about 1% of the nominal value by means of an electric signal. The use of absolutely measuring gap sensors is disadvantageous in that absolutely measuring gap sensors with a small offset error and very high measuring precision are expensive and require a high outlay for adjustment. The constantly present offset error of the gap sensors means that the rotor is displaced from its force-free floating state. This results in a constantly flowing current to compensate the offset error.
The use of more economical gap sensors is linked with the disadvantage that these have a larger offset error. On the one hand, with large offset errors of the gap sensors a very large current flows, which in some circumstances can exceed the power limits of the control device. On the other hand, a position displacement of the rotor from its force-free floating state takes place, so the catch bearings may be approached too closely or there may even already be contact in the extreme case.